Hot runner injection nozzles having torpedoes or valve stems in the melt stream typically create weld line blemishes in the finished part caused by the melt stream being divided by these obstructions and having to reform downstream thereof. The present invention provides an improved injection nozzle and method which includes an improved flow channel geometry to eliminate or significantly minimize these weld lines, while at the same time permitting faster color change performance.
When plastic melt flows through a hot runner system en route to a mold cavity, it sometimes must separate from a single solid cylindrical flow mass to pass by obstructions, such as torpedos, support fins and blades, valve stems, stem guides or support blades. When thus divided, the disturbed melt stream recombines downstream of the obstruction and there forms at least one weld line as the melt streams from the divergent paths come back together. Such a weld line, unless remixed homogeneously, thereafter continues to be present in the melt stream and appears as a blemish or line in the molded part formed from the mold cavity. Also when changing color of the melt considerable amounts of resin are wasted in flushing out the old color that is caught or stuck to these flow obstructions.
U.S. Pat. No. 4,266,723 to Osuna-Diaz and U.S. Pat. No. 4,279,588 to Gellert are examples of heat conducting torpedos. German Patent DE 32 49 486 to Manner and European Patent 0 638 407 to Krummenacher show valve stems being guided by nozzle tips having fins extending into the melt stream. All of these are examples of melt flow obstructions.
U.S. Pat. No. 5,405,258 to Babin shows a hot runner nozzle having a torpedo which is used to conduct heat absorbed from the upstream melt along its length to the gate area. The torpedo is positioned within the melt stream and supported by spiral blades that induce a swirling motion to the melt as it flows past them to help provide a stronger product in the gate area. The melt stream divides from a cylindrical stream upstream of the torpedo to an annular stream to pass the torpedo. It is also subdivided in the said annular stream to pass either side of the multiple spiral blades. Downstream of the blades the melt recombines briefly in the annular channel forming weld lines that may appear as lines in the molded part.
U.S. Pat. No.5,849,343 to Gellert et al. shows a valve gated nozzle having a stem guiding nozzle tip that causes the melt to divide from a cylindrical flow to annular flow first to negotiate the valve stem, then to divide again to negotiate the spiral fins supporting the stem at the tip. As in the '258 torpedo version, the melt must subdivide to pass by the spiral blades and recombine again.
The recombination of the flow in all of the foregoing examples typically causes weld lines to appear in the molded part; also color changing becomes lengthy and expensive.
Several attempts at mixing the melt in hot runner nozzles are shown in U.S. Pat. No. 4,965,028 to Maus et al., U.S. Pat. No. 5,513,976 to McGrevy, European Patent 0 546 554 to Gellert, and German Patent DE 32 01 710 to Gellert. A spiral fluted mixer included in an injection molding machine nozzle is shown in Austrian Patent 231696 to Hehl. All of these examples subdivide the melt stream into discrete paths that must recombine downstream of the obstruction and this tends to create flow blemishes in the molded part.
U.S. Pat. No. 5,545,028 to Hume et al. shows a hot runner tip having a semi-torpedo style in which the outer surface of the torpedo includes a flow channel that converts a single cylindrical inlet flow to an annular flow passing by the tip. However, the '028 patent does not show: grooves on the internal side of the tip body, any initial gap or clearance to eliminate hang-ups, a relaxation zone to reduce residual stresses from flow conversion in a grooved section, or a flow restriction or conformation zone to reduce the thickness variation generated in a grooved zone.
In spiral mandrel dies used in extrusion molding, single or multiple incoming cylindrical melt streams can be converted to a single annular outflowing stream in a continuous process like blown film extrusion molding. U.S. Pat. Nos. 5,783,234 and 5,900,200 to Teng show one application of this in a hot runner valve gated nozzle in which the spiral elements are formed in a comparatively large diameter valve stem and positioned relatively distant from the mold cavity gate. Reference should also be had to the following references: "Analysis for Extrusion Die Design" by B. Proctor, SPE ANTEC, Washington, D.C., pages 211-218 (1971); "The Nuts and Bolts of Blown-Film Design" by C. Rauwendaal, Plastics World, pages 85-87 (1991); and "Extrusion Dies for Plastics and Rubber" by W. Michaeli, Carl Hanser Verlag, Munich, ISBN 3-446-16190-2 (1992).
It is, therefore, a principal object of the present invention to provide an improved injection nozzle and method.
It is a further object of the present invention to provide an improved nozzle and method as aforesaid which eliminates or significantly minimizes weld lines in the finished product.
It is a still further object of the present invention to provide an improved nozzle and method as aforesaid which permits fast color change performance.
Further objects and advantages of the present invention will appear hereinbelow.